1. Field of the Invention
This invention relates to graphic display devices such as those used as terminal devices for data processing systems, wherein a central computer supplies data to be graphically displayed, e.g., upon a cathode ray tube.
2. Description of the Prior Art
In a known type of graphic display device, video signals defining alphanumeric or graphic data are stored in a pattern generator, and, on instruction from an external computer, desired patterns are individually read out for display on a cathode ray tube (CRT) by means of a raster scanning system which draws the patterns, raster by raster, into display matrices sized to correspond to one character space.
In such known devices, the character matrices on the CRT have their positions fixed, and thus the display position of a stored pattern is also fixed. Elevating or lowering a character position, e.g. to form a subscript or superscript, or elevating or lowering a graphic element, e.g., to fill in a graph, has required that the pattern generator store a number of video signals which represent various positional states to be assumed by a pattern. A significantly larger memory is required but the larger memory does not increase the variety of patterns available for display. For various reasons a satisfactory solution to the problem of shifting a pattern display position, so that it overlaps two display matrices, has not been found.